1. Field of the Invention
The present invention relates to electrophotographic photoconductors, and more particularly to electrophotographic photoconductors comprising an electroconductive support and a photoconductive layer comprising a particular bisazo pigment as a charge generating material which generates charge carriers when exposed to light.
2. Discussion of Background
Conventionally, a variety of inorganic and organic electrophotographic photoconductors are known. As inorganic electrophotographic photoconductors, there are known, for instance, a selenium photoconductor, a selenium-alloy photoconductor, and a zinc oxide photoconductor which is prepared by sensitizing zinc oxide with a sensitizer pigment and dispersing the same in a binder resin. Furthermore, as a representative example of organic electrophotographic photoconductors, an electrophotographic photoconductor comprising a charge transporting complex of 2,4,7-trinitro-9-fluorenone and poly-N-vinylcarbazole is known.
However, while these electrophotographic photoconductors have many advantages over other conventional electrophotographic photoconductors, they have several shortcomings from the viewpoint of practical use.
For instance, a selenium photoconductor, which is widely used at present, has the shortcomings that its manufacturing conditions are so difficult that its production cost is high, and it is difficult to work it into the form of a belt due to its poor flexibility. Furthermore, it is so vulnerable to heat and mechanical shocks that it must be handled with the utmost care.
In contrast to this, the zinc oxide photoconductor is inexpensive since it can be produced more easily than the selenium photoconductor. More specifically, it can be produced by simply coating inexpensive zinc oxide particles on a support. However, it is poor in its photosensitivity, and mechanical characteristics such as surface smoothness, hardness, tensile strength and wear resistance. Therefore, it is not suitable for a photoconductor for use in plain paper copiers in which the photoconductor is used in quick repetition.
The photoconductor employing the aforementioned charge transporting complex of 2,4,7-trinitro-9-fluorenone and poly-N-vinylcarbazole is also poor in its photosensitivity and therefore not suitable for practical use, particularly for a high speed copying machine.
Recently, extensive studies have been done on the electrophotographic photoconductors in order to eliminate the above-mentioned shortcomings of the conventional photoconductors. In particular, attention has been focused on a multi-layered type organic electrophotographic photoconductor, each comprising an electroconductive support, a charge generation layer comprising an organic pigment formed on the electroconductive support, and a charge transport layer comprising a charge transporting material formed on the charge generation layer, which are for use in plain paper copiers, since such multi-layered type organic photoconductors have high photosensitivity and stable charging properties as compared with the conventional organic photoconductors. Several types of the multi-layered type organic electrophotographic photoconductors are being successfully used in practice. Examples of the multi-layered type organic electrophotographic photoconductors are as follows:
(1) A multi-layered type electrophotographic photoconductor whose charge generation layer is prepared by vacuum evaporation of a perylene derivative and whose charge transport layer comprises an oxadiazole derivative, disclosed in U.S. Pat. No. 3,871,882.
(2) A multi-layered type electrophotographic photoconductor whose charge generation layer is prepared by coating an organic amine solution of Chlorodiane Blue on an electroconductive support and whose charge transport layer comprises a hydrazone derivative, disclosed in Japanese Laid-Open Patent Application No. 55-42380.
(3) A multi-layered type electrophotographic photoconductor whose charge generation layer is prepared by coating on an electroconductive support an organic solvent in which distyrylbenzene-type bisazo pigment is dispersed and whose charge transport layer comprises a hydrazone compound, disclosed in Japanese Laid-Open Patent Application No. 55-84943.
As previously mentioned, these multi-layered type electrophotographic photoconductors have many advantages over other conventional photoconductors, but at the same time, they have various shortcomings.
Specifically, the electrophotographic photoconductor (1) employing a perylene derivative and an oxadiazole derivative produces no problem when used in an ordinary electrophotographic copying machine, but its photosensitivity is insufficient for use in a high-speed electrophotographic copying machine. In addition to the above, since the perylene derivative, which serves as a charge generating material to perform the function of controlling the spectral sensitivity of the photoconductor, does not necessarily have spectral absorbance in the entire visible light range, this photoconductor is not suitable for use in color copiers.
The electrophotographic photoconductor (2) employing Chlorodiane blue and a hydrazone compound exhibits comparatively good photosensitivity. However, to prepare this photoconductor, an organic amine, for example, ethylene diamine, which must be handled with the utmost care, is essential as a solvent for forming the charge generation layer.
The electrophotographic photoconductor (3) employing a distyryl benzene type bisazo compound and a hydrazone compound have an advantage over other conventional electrophotographic photoconductors in that the charge generation layer can be prepared easily by coating a dispersion of the bisazo pigment on an electroconductive support. However, its photosensitivity is too low to be used in a high-speed electrophotographic copying machine.
In addition to the above photoconductors, various electrophotographic photoconductors employing particular azo compounds, which are considered to effectively work as a charge generating material when used in a multi-layered type electrophotographic photoconductor, are proposed. For example, a benzidine-type bisazo compound is disclosed in Japanese Laid-Open Patent Applications 47-37543 and 52-55643; a stilbene-type bisazo compound is disclosed in Japanese Laid-Open Patent Application 52-8832; a diphenylhexatriene-type bisazo compound is disclosed in Japanese Laid-Open Patent Application 58-222152; and a diphenylbutadiene-type bisazo compound is disclosed in Japanese Laid-Open Patent Application 58-222153.
However, the multi-layered type electrophotographic photoconductors employing the above-mentioned conventional azo compounds do not satisfactorily function in the high-speed copying machine, since the photosensitivity thereof is poor.
Recently, there is a demand for an electrophotographic photoconductors for use in a laser printer, in particular, for photoconductors having high photosensitivity in a semiconductor laser wavelength region. The photosensitivity of the above-mentioned photoconductors are too low to be put to practical use for semiconductor laser.
The applicants of the present application have already proposed a diphenylhexatriene-type bisazo compound, which is disclosed in Japanese Laid-Open Patent Applications 58-222152 and 59-136351. As a result of further extensive studies, it is found that the photosensitivity of the above diphenylhexatriene-type bisazo compound is remarkably increased by introducing a substituent into a benzocarbazole ring of the compound at the 8-position, and then the present invention is attained. Although Japanese Laid-Open Patent Application 62-147463 describes the introduction of a substituent into the benzocarbazole ring, it does not refer to the diphenylhexatriene-type bisazo compound as an azo component.